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How to send Sensor Output signlas to Long distnaces??

Govind K
Intellectual 510 points
Other Parts Discussed in Thread: F28M35H52C, SN65HVD74, LM331

Hello ,

In want to interface Incremental encoder senosor output to Microcontroller which is 10Meters away from the sensor.

Specifications:

Sensor Output : Square wave 0V to 5V , 100KHz frequency. 

Controller: F28M35H52C, EQEP module interface.

As per my understanding, the avaialable options are:

1. Convert sensor output to differential ended signals using RS422/LVDS etc....

2. Convert sensor output to differential ended signals using OP Amps  etc....

Is my approach correct,..?

Kinldy suggest the best method(s).

Thanks in advance.

Regards,

Govind.

  • 0
    TI__Expert 3045 points

    Hi Govind,

    Suggest use the 1st options.

    I noticed that the signal is a low speed, digital signal. Op amps can be used flexibly in converting single-ended to differential, and it is true that you can use op amps to realize your transmission. However, it is not recommended to use op amps in your application.

    Firstly, op amps it is widely used in analog signal transmission,not digital signal. Using digital signal transceivers, such as RS-422/485 converters is quite easy: easy to design, easy to layout, easy to design, etc.

    Secondly, if you connect the output of an op amp directly to a long distance wire, the capacitance of the wire may cause the instability of the op amps. RS422/485 transceivers are specially designed in these occasion so that it is stable enough in these applications.

    Also, integrated transceivers, especially transceivers released in the recent years, have some awesome features such as ESD protection, power consumption saving mode, etc.

    So you can refer to the selection table for more details:

    http://www.ti.com/lsds/ti/interface/rs-422-products.page

  • 0 in reply to Vio Liu
    Intellectual 510 points
    Hello Vio Liu,

    Thanks for reply. I completely agree with your answer. I am already using SN65HVD74, RS422 transceiver in my project, so i could use the same here.

    But, your second point has raised a new concern for my other interface circuit. Details are mentioned below.

    I have an Analog Hall effect sensor which located at 10 meters distance (as above) away from my controller.
    Specs are: Sine wave whose magnitude varies from 1V to 4V . Maximum frequency 300Hz.

    Till now my idea is to add op- amp buffers at sensor end and transmit through shielded cables (Not identified yet) to the controller. And then level shifting using voltage divider at ADC inputs (3.3V ADC reference).

    Is it a good method to implement?? as you mentioned, cable capacitance might create some issues in the OP AMP..??
    Kindly suggest reliable solution.

    Thanks & Regards,
    Govind.
  • 0 in reply to Govind K
    TI__Expert 3045 points

    Hello Govind,

    You mentioned that you want to transmit a signal of 300Hz to 10m away. Actually the frequency is quite low, but the distance is a little long. When long distance/high frequency happens, we can use such kind of topology:

    Rm is the characteristic impedance and Rt is the terminal resistor. With these two resistors, we can make sure that there's no signal reflection.

    Rm should be equal to the characteristic impedance of the cable.

    Also you mentioned the capacitance. If any capacitance exists, we can model it like below:

    and you can evaluate how much the capacitance is.

    We can use this circuit to show the model of op amps driving a capacitance load.

    In this case, Riso should be at least 4/(2*PI*f*C)

    PI is the Archimedes constant, f is GBWP, and C is the total capacitance value of the cable.

    So you can calculate each value and use the topology shown above.

    Be be honest, the most important issue you may encounter is probably not the signal transmission. The power frequency interference can make your signal totally hidden in the 50Hz/60Hz signal. Suggest to use the differential signal, or, if possible, convert the analog signal into digital signal and transmit through 485/422 bus.

    Also, a good idea is that you can use VFC such LM331 to convert the analog signal to digital signal and transmit the signal to your controller.

    Thank a lot for your reply.

    Best regards!